1. Field of the Invention
The present invention relates to a lead frame for semiconductor device, having a protective coating for improving the reliability of wire bolding to the lead frame.
2. Description of Related Art
Lead frame is a thin metal plate produced by punching a metal strip of copper or copper alloys, and includes die pads on which semiconductor chips are fixed, inner leads to be connected to semiconductor chip, and outer leads to be connected to printed circuit board. Semiconductor devices, such as transistors and integrated circuits, are manufactured by the processes of fixing semiconductor chips on the die pads of the lead frames, connecting each of bonding pads (electrodes) of the chips to the corresponding one of the inner leads of the lead frames by means of wire bolding, and forming packages to cover the chips. In the conventional wire bolding, fine wires of gold or aluminum are mainly used as bonding wires.
The surface of the lead frame is easily oxidized by contact with air, and the bonding strength between the bonding wires and the inner lead decreases as the oxide layer grows thicker. In such cases, the risk arises that defective semiconductor device will be increased drastically.
One of the effective means to avoid such a risk is to carry out the wire bonding in a reducing atmosphere. However, this method requires not only expensive reducing gas, but also demands complicated operations for maintaining a suitable reducing atmosphere.
Another means for preventing the drop in bonding strength by the oxidation is to form a gold layer on the surface of the lead frame by electroplating. In this case, the thickness of the protective coating must be more than several microns, because electroplated gold layers of less than several microns cannot provide sufficient effect for preventing the oxidation of the surface of the lead frame. Therefore, a considerable amount of gold is required for electroplating, and the manufacturing cost of lead frame becomes expensive.
Furthermore, in the conventional lead frame, in order to facilitate soldering of the outer leads to printed circuit board, it is necessary to form a solder layer on the outer lead by means of electroplating or solder dipping. However, such a process for forming solder layer requires a pretreatment for removing oxide layer of the outer leads, therefore, regarding this point, the conventional lead frame has a drawback that the manufacturing process thereof is complicated and manufacturing cost is high.